it is the size of area where the head is in and you cant see it any more......:) ;)
When two [[Physical body|objects]] touch, a certain portion of their surface areas will be in contact with each other. '''Contact area''' refers to the fraction of this area that consists of the [[atom]]s of one object being in contact with the atoms of the other object. Because objects are [[asperities|rarely perfectly flat]], the actual contact area (on a [[microscopic]] scale) is usually much less than the contact area apparent on a [[macroscopic]] scale. Contact area may depend on the [[normal force]] between the two objects due to [[deformation]].
The contact area depends on the geometry of the contacting bodies, the load, and the material properties. The contact area between two parallel cylinders is a narrow rectangle. Two, non-parallel cylinders have an elliptical contact area, unless the cylinders are crossed at 90 degrees, in which case they have a circular contact area. Two spheres also have a circular contact area.
==Friction and Contact Area==
It is an [[empirical]] fact for many [[materials]] that F = μN, where F is the [[friction|frictional force]] for [[sliding friction]], μ is the [[coefficient of friction]], and N is the [[normal force]]. There isn't a simple derivation for sliding friction's independence from area.
==Methods for Determining Contact Area==
One way of determining the actual contact area is to determine it indirectly through a physical process that depends on contact area. For example, the [[Electrical resistance|resistance]] of a wire is dependent on the cross-sectional area, so one may find the contact area of a metal by measuring the [[Electric current|current]] that flows through that area (through the surface of an [[electrode]] to another electrode, for example.)
